Super-pulsed CO2 slab lasers are commonly used in the industry for a variety of material processing applications, such as drilling via holes in printed circuit boards. When operating such a laser, an RF pulse 100 is typically applied to the laser electrodes at time t1 as shown in FIG. 1(a). A high peak RF power Pp is applied over a period of time TRF. The RF pulses used to drive CO2 slab lasers, for example, typically have peak powers on the order of 10 kW to 20 kW, often around 15 kW. The peak powers are approximately equal to the average power capability of the RF power supply divided by the duty cycle. The pulse widths are typically between 20 μs and 100 μs, often around 55 μs, with a low duty cycle between 5% and 10%. Pulse repetition rates are typically between 500 Hz to 4.0 kHz.
The application of the RF pulse to the electrodes causes an optical pulse 110 to be generated by the slab laser, as known in the art, having typical output pulse characteristics as shown in FIG. 1(b). After a laser build-up time TB, normally 5 μs to 10 μs for various systems, and at time t2, the laser gas medium reaches an oscillating threshold and begins to oscillate. These oscillations occur first with a high peak power, gain-switched spike 112, followed by a relatively slow rise 114 in laser power until full laser output power 116 is reached. This relatively slow rise time can be detrimental to hole drilling quality, and can affect the shape and/or size of a drilled hole. When the RF excitation pulse is turned off, at time t3, the output of the laser begins an exponential decline 118 as the gain in the medium dissipates, the rate of decline being determined by the Nitrogen/CO2 gas mixture kinetics. This decay constitutes the long tail of each output pulse, as shown in FIG. 1(b) after time t3. The energy in this tail of the laser pulse can degrade application quality, such as the shape of drilled via holes.
For applications utilizing these laser systems, it can be desirable to increase the pulse repetition rates to obtain increased throughput. Many of these applications are prevented from operating at these higher rates, however, as the application equipment, such as a galvanometer-based laser scanner, is presently not able to operate at these higher pulse repetition rates. For instance, a typically galvo scanner can have an upper scanning rate between about 1 kHz and about 2 kHz, which is lower than the pulse repetition rate capability of slab lasers. Many applications also would benefit from a clipping of the slow rise time 114 and long, decaying tail 118 at the beginning and end of each laser pulse, as it can be desirable for these applications to obtain a fast rise and/or fall time for these laser pulses. In a hole drilling application, for example, a faster fall time and/or faster rise time for a laser pulse can improve the quality of a drilled hole.
Presently, CO2 slab laser system manufacturers attempt to adjust the shape of the slab laser pulses using acousto-optic modulators, such as to clip the slow rise time of the laser pulses, clip the long decaying tail in the back of the laser pulses, or both. Acousto-optical switches also are utilized to direct laser pulses to different locations. A problem with acousto-optic devices, however, is that there is a significant amount of power loss and beam distortion caused by the relatively large optical absorption existing in these present day Ge-based devices. These optical losses yield 15% to 20% optical losses, and lead to a degradation of laser beam quality due to the heating of the Ge crystal by the laser beam. This degradation of laser beam quality results in inferior hole quality.